1. Field of the Invention
This invention relates to a lanthanum chromite material which can be advantageously used as an interconnector material in electrochemical cells such as solid-oxide fuel cells and solid-electrolyte steam electrolyzers.
2. Description of the Related Art
For example, in solid-oxide fuel cells (hereinafter abbreviated as SOFCs) or solid-electrolyte fuel cells, interconnectors are used as connecting members for combining a plurality of single cells to raise the total voltage and thereby obtain high electric power. Such interconnectors function not only to provide electrical connection, but also to separate the oxidizing gas (air) from the reducing gas (fuel) at elevated temperatures. Accordingly, refractory metals having a high melting point and perovskite type oxides, such as lanthanum chromite doped with Mg, Ca or Sr, have been used as interconnector materials.
Since the operating temperature of SOFCs is as high as 1,000.degree. C. or so, even high-melting metals form an oxide during long-term use in an oxidizing atmosphere. This is undesirable in that their surfaces come to have electrical insulating properties and thus exhibit poor electric conduction.
On the other hand, lanthanum chromite which is an oxide is stable in an oxidizing atmosphere, but has the disadvantage that its electric conductivity is low and some characteristics thereof, such as electric conductivity, change as a result of reduction in a reducing atmosphere. For this reason, lanthanum chromite has been doped with an alkaline earth metal, such as Mg, Ca or Sr, in order to enhance its electric conductivity. When lanthanum chromite is doped with Mg, Ca or Sr, its electric conductivity becomes higher in the Order mentioned.
Moreover, since an SOFC is an assembly consisting of a solid electrolyte [i.e., YSZ (Y.sub.2 O.sub.3 -stabilized ZrO.sub.2 or yttria-stabilized zirconia)], electrodes, such as oxygen electrodes and fuel electrodes, and interconnectors, the coefficient of thermal expansion of the interconnectors needs to match with that of YSZ, which forms a base. From this point of view, lanthanum chromite doped with Sr has a coefficient of thermal expansion of the order of 10.degree..times.10.sup.-6 .degree. C..sup.--1 which is almost equal to that of the solid electrolyte YSZ. Accordingly, lanthanum chromite doped with Sr which has a high electric conductivity and a coefficient of thermal expansion almost equal to that of the solid electrolyte YSZ has been used as an interconnector material for SOFCs.
Observing the behavior of lanthanum chromite doped with Sr in a reducing atmosphere, however, it has been found that a relatively high degree of expansion is caused by reduction. This is presumed to be responsible for deformation and cracking of the interconnectors, and delamination of the electrodes.